Polyoxymethylene polymers, having recurring units of the formula --CH.sub.2 O-- have been known for many years. They may be prepared by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane, which is a cyclic trimer of formaldehyde. Polyoxymethylene varies in thermal stability and in molecular weight, depending on its method of preparation.
Polyoxymethylene of high molecular weight and thermal stability is useful in the production of molded and extruded objects. A polyoxymethylene copolymer having improved resistance to thermal degradation is disclosed in commonly-assigned U.S. Pat. No. 3,027,352. This copolymer contains a major proportion (i.e., at least 85 percent) of oxymethylene units. This copolymer yields tough, heat-resistant, dimensionally stable molded products of high strength. The impact resistance is good, but for certain purposes (e.g., for automobile body and trim parts, such as fender parts, and for machine housings) an even higher resistance to impart is advantageous.
The addition of polyhydric alcohols such as glycerol or trimethylol propane or of readily volatile substances such as trioxane or 1,4-dioxane slightly increases the impact resistance of polyoxymethylenes but also involves a sharp reduction of rigidity and tensile strength. Basically the same applies to the addition of carboxamides. Furthermore, the above substances suffer from the added drawback that they can be washed out of the shaped articles or exude therefrom.
It has also been proposed to prepare thermoplastic molding compositions by blending polyoxymethylene with high molecular weight polyurethanes obtained from polyhydroxy compounds having molecular weights of from 500 to 4000, polyisocyanates and chain lengtheners. Here again, the rigidity and tensile strength suffer a strong reduction. Furthermore, the stability of the products is impaired, particularly as regards discoloration caused by thermal stresses and light.
Commonly-assigned U.S. Pat. No. 3,476,832 recommends the addition to oxymethylene polymers of butadiene/acrylonitrile copolymers, polyacrylates or copolymers of diolefins with alkylmethacrylates. However, these additives tend to reduce the thermal stability of the molding compositions and cause discoloration during processing.
U.S. Pat. No. 3,749,755 discloses thermoplastic molding compositions of improved impact resistance based on a mixture of an oxymethylene polymer and an elastomeric graft copolymer. The elastomeric graft copolymer comprises a prepolymer made from 10 to 99 percent by weight of an acrylic ester; 1 to 90 percent by weight of a diene; and, optionally, up to 25 percent by weight of at least one additional monomer. To the prepolymer is grafted styrene, methyl methacrylate, or mixtures thereof, and, optionally, acrylonitrile, methacrylonitrile, acrylic esters, vinyl esters, vinyl ethers, vinyl halides, or vinyl-substituted heterocyclic compounds.
U.S. Pat. No. 3,975,459 discloses thermoplastic molding compositions of high impact resistance based on polyoxymethylenes and block copolymers of a diene polymer block and at least one poly (alkylmethacrylate) block.
U.S. Pat. No. 4,181,685 discloses thermoplastic molding compositions in which the crystalline structure of oxymethylene polymers is modified by the addition of branched or cross-linked polyoxymethylenes as nucleating agents. The branched or cross-linked polyoxymethylenes include multifunctional reactive compounds. The compositions are not disclosed as having any beneficial effect upon the impact strength of the oxymethylene polymers.
Thus, it is an object of the present invention to provide a blend of oxymethylene copolymers having improved impact resistance.
It is also an object of the present invention to provide a blend of oxymethylene copolymers which exhibits improved impact resistance with no significant reduction in tensile and flexural properties.
It is also an object of the present invention to provide a blend of oxymethylene copolymers which exhibits improved impact resistance, toughness, and flexibility.
These and other objects as well as the scope, nature, and utilization of the present invention will be apparent from the following description and appended claims.